1. Field of the Invention
The invention concerns a dividing wall of the type made from at least one wall material for delimitation of a patient positioning region from an antenna structure of a magnetic resonance tomography apparatus. Moreover, the invention concerns a magnetic resonance tomography apparatus with such a dividing wall.
2. Description of the Prior Art
Magnetic resonance tomography is a widespread technique for acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this method, the body or a body part of the patient or subject to be examined must initially be exposed to an optimally homogeneous static basic magnetic field which is generated by a basic field magnet of the magnetic resonance system. During the acquisition of the magnetic resonance images, rapidly-switched gradient fields for spatial coding, which gradient fields are generated by gradient coils, are superimposed on this basic magnetic field. Moreover, radio-frequency pulses of a defined field strength are radiated into the examination subject with radio-frequency antennas. The nuclear spins of the atoms in the examination subject are excited by means of these radio-frequency pulses so that they are deflected from their equilibrium position (parallel to the basic magnetic field) by what is known as an “excitation flip angle”. The nuclear spins then precess in the direction of the basic magnetic field. The magnetic resonance signals thereby generated are acquired by the radio-frequency acquisition antenna. The magnetic resonance images of the examination subject are ultimately generated on the basis of the acquired magnetic
A typical magnetic resonance tomography apparatus comprises a patient positioning region (also called a patient chamber in the following) in which is located a patient bed on which the patient is positioned during the examination. For example, this can be what is known as a “patient tunnel” in a tube running through the housing of the scanner of the tomography apparatus. Moreover, there are also MR systems with a patient positioning region opening laterally that is enclosed in a U-shape by the housing of the MR scanner. Strong coils for generation of the permanent magnetic field are located within the housing of the scanner. Moreover, further coils for generation of the gradient fields are located within the housing.
Furthermore, the scanner typically has an antenna structure permanently installed in the housing, with which antenna structure the required radio-frequency pulses are emitted into the patient positioning region and with which excited magnetic resonance signals can be acquired. This radio-frequency antenna is also known as a “body coil”. Moreover, coils known as “local coils” are additionally e/d in many magnetic resonance tomography systems. Such local coils can be placed directly on or below the regions of the body of the patient in whom magnetic resonance exposures are generated. These local coils can likewise be used both as a transmission antenna and as am acquisition antenna. In many examinations, however, the RF pulses are emitted from radio-frequency antennae integrated into the magnetic resonance examination scanner and the excited MR signals are received with the local coils.
An MR radio-frequency antenna is typically formed by a resonant antenna structure that includes inductances (for example a conductor structure) and various capacitances. In principle high potential jumps occur at the capacitances (which, for example, are formed by capacitors that couple the individual parts of the conductor structure with one another). The aforementioned radio-frequency antenna (the body coil) integrated into the magnetic resonance scanner itself is typically arranged directly on or near to the dividing wall described above. The dividing wall demarcates the patient chamber from the radio-frequency transmission antenna. This dividing wall normally simultaneously forms the inner wall of the housing of the magnetic resonance scanner that faces the patient space. In the case of a patient tunnel this dividing wall is, for example, the tube wall that lines the patient tunnel from the inside. Contact of the patient with the dividing wall therefore can lead to capacitive couplings of the electrical fields (E-fields) from the antenna into the body tissue of the patient, which can lead to high local SAR exposures (SAR=Specific Absorption Rate). Together with the body surface, the antenna structure thus forms a type of plate capacitor with the dividing wall as a dielectric therebetween.
In order to reduce or avoid the coupling of E-fields from the antenna into the body tissue as much as possible, it is sought by suitable positioning aids to avoid a direct contact by the patient. Alternatively, in some situations a larger radial separation of the antenna structure from the dividing wall makes the E-field coupling lower. This requires a greater structural height of the scanner, which leads to significantly higher costs.